The present invention relates generally to automatically resetting a surge protection circuit, and specifically to systems and methods for automatically resetting a surge protection circuit within a telephone line interface circuit after receiving a transient energy surge, such as lightning or an AC voltage spike, by disabling and then re-enabling the surge protection circuit based on discharge current.
Telephone line interface circuits are electronic circuits that service the conventional two-wire (the tip and ring wires) telephone subscriber lines. Active telephone line interface circuits, in which the tip and ring wires of a telephone subscriber are driven via active line circuits, are well known in the art. Exemplary active telephone line interface circuits are described in U.S. Pat. No. 5,274,702 issued to Rosch et al. on Dec. 28, 1993 entitled xe2x80x9cWideband Telephone Line Interface Circuitxe2x80x9d and in U.S. Pat. No. 5,323,461 issued to Rosenbaum et al. on Jun. 21, 1994 entitled xe2x80x9cTelephone Line Interface Circuit With Voltage Switching.xe2x80x9d
As described in the above-referenced patents, a line drive circuit in a telephone subscriber line interface circuit typically includes two unity-gain DC amplifiers whose outputs are coupled to the tip and ring wires, respectively, of the telephone subscriber line via a switching circuit and a sensing circuit. The telephone line interface circuit also includes a controlled voltage generator (more generally referred to as a power source) and a control circuit. The sensing circuit serves to monitor AC and DC conditions on the line and includes a sensing transformer and series resistors, which are generally referred to as feed resistors. The switching circuit serves, in association with software control via the control circuit, to interconnect the line, the line drive circuit, the controlled voltage generator, and a battery in various configurations to provide for different operating states of the line interface circuit. These states include configurations in which relay contacts connect the outputs of the line drive circuit or the output of the controlled voltage generator selectively to the tip and ring wires of the line, and in which relay contacts and an electronic switch selectively connect the output of the controller voltage generator or the battery as a supply voltage for the line drive circuit.
It is also well known to protect the line drive circuit in an active telephone line interface circuit from high transient voltages and currents (more generally referred to as transient energy surges) due to lightning, AC surges, and inductive coupling. To this end, it is known to couple a front-end protection circuit, such as a diode bridge, to points between the outputs of the line drive circuit and the feed resistors. As a front-end protection circuit, the diode bride clamps positive transient voltages to ground and clamps negative transient voltages to a breakdown voltage of a transient voltage suppressor (such as a thyristor), which is connected between a negative supply voltage and ground. In order to provide sufficient power dissipation for negative transient voltages or currents, a common solution uses several transient voltage suppression diodes (frequently referred to by the trade name TRANZOR(trademark) or TRANSIL(trademark) from SGS-Thomson Microelectronics) in combination. However, such a solution has the disadvantages of incurring a relatively high and undesirable cost. Additionally, such a solution requires a relatively large amount of physical space.
It would be desirable to be able to replace the transient voltage suppression diodes by a single crow-bar type protection device, frequently referred to by the trade name TRISIL(trademark) or SIDAC(trademark). However, such a device has the disadvantage that, if used in an active telephone line interface circuit, the current supplied by the line drive circuit can be sufficient to maintain or hold the protection device operative at and after the end of a negative transient voltage or surge. This commonly results in a locked up state of the telephone line interface card. In other words, the surge protection circuit within the telephone line interface circuit protects the card from damage in an inexpensive and space saving manner, but may possibly leave the card in an inoperable or locked up state.
The applicant discovered and patented an improved telephone line interface circuit aimed at avoiding these disadvantages. In particularly, U.S. Pat. No. 5,539,820 (hereinafter the 820 patent) issued on Jul. 23, 1996 to the applicant and is entitled xe2x80x9cProtection of Active Telephone Line Interface Circuitsxe2x80x9d describes surge protection circuitry used for protecting the telephone line interface circuit from transient energy surges and is hereby incorporated by reference. As can be seen in the 820 patent, the surge protection circuitry uses a current limiter portion to protect a power supply in the interface and a front-end protection portion to protect against positive and negative transient voltages and currents.
In more detail, the front-end protection portion is described as a clamping diode, which is connected to a return on the telephone line interface circuit""s power source. In this configuration, the clamping diode channels positive surge currents to the return when the potential rises to a diode drop above the return. The transient voltage suppression portion of the front-end protection circuitry is described as one or more specially designed diodes, such as a series combination of a thyristor and a clamping zener device. These diodes are designed to clamp excess voltages and to dissipate high transient power. When a negative voltage caused by a surge exceeds a predetermined threshold, the thyristor will breakover and create a path to the return for the negative surge current through the zenering device. In both instances (positive and negative surges), the surge current is diverted via the surge protection circuit to the return instead of to the power source where damage may occur.
Unfortunately, the applicant has found that in some situations, it is possible that the surge protection circuitry may become latched in an intermediate state. In particular, the applicant has found that the current supplied by the line drive circuit in intermittent situations may still be sufficient to maintain the operation of the surge protection circuit after the end of a negative transient voltage or surge without resetting back to a nominal state. In those rare but frustrating situations, continued use of the line interface circuit is not possible. While the surge protection circuit successfully protects the telephone line interface circuit, the protection circuit remains operative and undesirably blocks normal operation of the telephone line interface circuit.
In light of the foregoing, a need exists for an improved technique and system that prevents lockup of the surge protection circuit within the telephone line interface circuit. A need also exists for an improved technique and system to quickly and automatically reset the surge protection circuit within the telephone line interface circuit.
Accordingly, the present invention is directed to a method and apparatus for resetting a surge protection circuit within a telephone line interface that substantially obviates one or more of the limitations and disadvantages of the described prior arrangements.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve these and other objects and advantages, and in accordance with the purpose of the invention as embodied and broadly described herein, a telephone line interface""s surge reset circuit consistent with the present invention includes a sensor and a feedback circuit. The sensor is connected to a discharge terminal of a surge protection circuit in the telephone line interface. The surge protection circuit usually includes a current limiter for protecting a power source in the telephone line interface and a transient voltage suppression circuit having the discharge terminal and being operatively connected to the current limiter. The sensor is typically a zener device that senses a discharge current that flows within the surge protection circuit through the discharge terminal. The feedback circuit is connected to the sensor and is capable of providing a reset feedback signal to the surge protection circuit. The reset feedback signal is provided in response to a voltage level detected across the sensor. More particularly stated, the voltage level detected is a voltage level across the zener device as the discharge current flows through the zener device. In this manner, the reset feedback signal resets the surge protection circuit.
Additionally, the feedback circuit may include a feedback transistor having a base electrically connected to the sensor. The feedback transistor may turn on and provide the reset feedback signal to the surge protection circuit when the sensor detects the discharge current.
The surge protection circuit may also include a current limiter and at least one thyristor connected to the current limiter for protecting a power source in the telephone line interface from a negative surge. In such a situation, the feedback transistor may disable the current limiter within the surge protection circuit when the sensor detects the discharge current in order to reset the thyristor.
Furthermore, the surge reset circuit may also include a cutoff device capable of disabling the current limiter when receiving the reset feedback signal from the feedback transistor. The cutoff device may also enable the current limiter within the surge protection circuit when the discharge current is below a threshold value and the reset signal is removed.
In another aspect, a surge reset circuit for use with a surge protection circuit in a telephone line interface and consistent with the present invention has a sensor connected to a discharge terminal of a first portion of the surge protection circuit. The sensor may be a nonlinear device (such as a zener device) that limits the voltage level across the nonlinear device when the nonlinear device is biased with current flowing through the current discharge terminal. The zener device typically has a characteristic limiting voltage value representing a maximum voltage drop across the zener device.
A feedback circuit is connected to the sensor and is capable of providing a reset feedback signal to a second portion of the surge protection circuit in response to a voltage level generated by the sensor representing a discharge current flowing through the discharge terminal. The feedback circuit usually includes a feedback transistor having a base connected to the sensor and controlled by the detected voltage level across the sensor. The feedback transistor may turn on and provide the reset feedback signal to the cutoff device when the sensor detects the discharge current and generates the voltage level.
Finally, the surge reset circuit includes a cutoff device connected between the feedback circuit and the second portion of the surge protection circuit. The cutoff device is capable of resetting the surge protection circuit by disabling the second portion of the surge protection circuit (such as a current limiter) in response to the reset feedback signal. The cutoff device is typically capable of resetting the front-end protection circuit by reducing the discharge current through the discharge terminal while the sensor detects the discharge current. Additionally, the cutoff device is typically further capable of enabling the diode current limiter when the discharge current is substantially eliminated. The cutoff device is normally implemented as a power MOSFET device itself or the power MOSFET device and its associated controlling circuit.
In yet another aspect, a method consistent with the present invention is shown, for resetting a surge protection circuit within a telephone line interface. The method comprises the steps of receiving a transient energy surge within the telephone line interface, protecting a power source within the telephone line interface during the transient energy surge using the surge protection circuit, sensing an amount of current draining from the surge protection circuit, and automatically resetting the surge protection circuit for a next surge based upon the current draining from the surge protection circuit. Automatically resetting the surge protection circuit is typically done by disabling a current limiter portion of the surge protection circuit, reducing current flowing through a negative surge protection portion of the surge protection circuit, and enabling the current limiter portion of the surge protection circuit.
In more detail, disabling the current limiter portion is usually accomplished by generating a voltage level by a sensor (such as a zener device) that is configured between a return of the power source in the telephone line interface and a current discharge terminal of the negative surge protection portion of the surge protection circuit. A feedback circuit may then be activated to generate a reset feedback signal if the current is sensed to be draining. This is typically accomplished by turning on a feedback transistor within the feedback device in response to the generated voltage level across the zener device and then drawing a feedback current through the feedback transistor to generate the reset feedback signal. Once the feedback circuit is activated, the reset feedback signal may be provided to a cutoff device, which disables the current limiter portion.
Additionally, the draining step usually involves reducing the amount of current from the negative surge protection portion to reset the negative surge protection portion. In other words, the current flowing through the negative surge protection portion is reduced to reset the negative surge protection portion of the surge protection circuit. In this manner, the current limiter portion of the surge protection circuit may be enabled (typically by removing the reset feedback signal from a cutoff device) after the negative surge protection portion is reset.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.